Paints are typically formulated with a combination of pigments and extenders to achieve desired, hiding power, tint strength, gloss and sheen, color, scrub resistance, burnish resistance, and stain resistance. The selected extenders can, in some cases, economically extend the functionality of the pigments in the formulation. The pigments used in paint can include inorganic and organic pigments, pigment flakes, insoluble dyes and other durable coloring matter. While the pigmentation of the paint can be solely from prime pigments, this is economically impractical given the indicated high pigment volume concentration. Titanium dioxide is widely used in paints and coatings to improve brightness and opacity, but it is an expensive prime pigment. In recent years, considerable efforts have been made to develop satisfactory replacements for titanium dioxide. Common pigment extenders include calcium carbonate, barytes, magnesium silicates, aluminum silicates including calcined and hydrous kaolin, diatomaceous earth, silica and mica. For scrub, burnish, and stain resistance, extenders like nepheline syenite, albite, and ceramic microspheres find use.
Calcined kaolin in paint has traditionally found use in interior higher PVC flat and eggshell paints. Its functionality has been as an opacifier to extend more costly pigments and provide flatting to control the gloss development of the coating. These pigment extenders are produced by partially or fully calcining a fine particle size fraction of hydrous kaolin. Calcined kaolin clay opacifying pigment extenders, such as the products supplied under the trademarks Satintone™ 5 and Satintone™ 5HB by BASF Corporation are exemplary and have proven superior to other pigment extenders, e.g. calcium carbonate and hydrous kaolins.
On a commercial scale, kaolin calcination may be carried out in a rotary calciner with countercurrent flow of hot air or in a Nichols Herreshoff vertical furnace. In the laboratory, a muffle furnace is usually applied. Kaolin to be calcined is typically a finely dispersed powder with a Hegman grind of 4.5 or higher. This degree of dispersion is generally achieved by passing the dry kaolin powder through an appropriately designed pulverization process.
To one skilled in the art of kaolin calcination, kaolin, when heated, will undergo a series of crystalline form changes that offer significantly different physical and chemical property attributes. The first of these occurs in the 840 to 1200° F. (450°-650° C.) range. Here hydrous kaolin dehydroxylates with the formation of an amorphous essentially anhydrous material usually referred to as “metakaolin.”
As incremental heat is applied to metakaolin, its lattice will reconfigure to a gamma-alumina (spinel) phase. This typically occurs as the feed material reaches a temperature range of 1650 to 1750° F. (900 to 955° C.). Above this temperature, the gamma alumina incrementally converts to mullite. At 2300° F. (1260° C.), the conversion to mullite is essentially complete. At higher temperature, the synthetic mineral matrix will again reconfigure into cristobalite. X-ray diffractometry (XRD) is a convenient way to assess the level of mullite present in the spinel lattice. Mullite index (M.I.) is a quantitative x-ray diffraction method used to quantify the amount of mullite in a material. The quantification is done by integrating the area of a peak, or peaks, and comparing the integrated peak intensity of the unknown sample to a calibration curve. The calibration curve is typically formed by running samples consisting of 10% increments of mullite from 0% to 100%. Thus, a mullite index of 35 indicates that the sample contains about 35% mullite. Since mass absorption or preferred orientation typically are not taken into account, the mullite index value cannot exactly be termed as percent, but can be used in a relative sense as a useful percent range of mullite in the sample. In general, after calcination, the inert matrix typically has from 40-60% SiO2 and 60-40% Al2O3.
Calcined kaolin pigment extenders have been used for several decades in a number of industrial applications such as paper coating, paper filling, paints, plastics, etc. In these applications the kaolin pigment extenders impart to the finished products a number of desirable properties: TiO2 extension/opacity, sheen/gloss control, electrical resistivity, strength (in plastics), friction (in paper). Paper coating and filling applications almost exclusively require fine fully calcined kaolin pigments such as the 93% brightness ANSILEX®-93 pigment manufactured by BASF Corporation. See, for example, U.S. Pat. No. 3,586,523, Fanselow et al, which describes the production of such pigments from ultrafine Tertiary “hard” ultrafine kaolins. The term “fully calcined” is of interest because it defines a rather narrow range of calcined kaolin matrix structures. Calcination has been progressed into the spinel phase and arrested when only a small degree of mullite (10% by weight or less) has been incorporated.
The temperature at which the aforementioned crystalline transitions occur can be lowered by the addition of a flux to the hydrous kaolin feed before calcination as disclosed in commonly assigned U.S. Pat. No. 6,136,086. Reference is made to U.S. Pat. No. 2,307,239, Rowland, which is a pioneer patent in the field of calcined kaolin pigments. This patent broadly discloses addition of various alkali and alkaline earth compounds to clay before calcination.